1. Field of the Invention
The present invention relates to a surface-mount quartz crystal unit, and more particularly, to a surface-mount crystal unit which is suitable for reducing an area necessary to mount the crystal unit on a wiring board.
2. Description of the Related Art
A surface-mount crystal unit which has a quartz crystal blank encapsulated in a surface-mount container is frequently used as a frequency or time reference source, for example, in portable telephones because of its compact size and light weight. At present, typical surface-mount crystal units commercially available on the market have outer planar dimensions of 4.0 mmxc3x972.5 mm and 3.2 mmxc3x972.5 mm, and a surface-mount crystal unit now under investigation is further reduced in outer planer dimensions to 2.5 mmxc3x972.0 mm.
FIG. 1 illustrates a conventional surface-mount crystal unit. The illustrated crystal unit comprises concave container body 1 made of laminated ceramics having bottom wall 1a and frame wall 1b; crystal blank 2 contained in container body 1, and cover 3 which is overlaid on container body 1. Bottom wall 1a is substantially rectangular in shape, when viewed in a plan view, and frame wall 1b is also formed in a substantially rectangular shape frame, so that container body 1 is formed with a substantially rectangular recess. A pair of crystal-connecting terminals, not shown, are formed on an inner surface of the recess (i.e., inner bottom face) of container body 1. A pair of mounting electrodes 4 are formed from side faces to a bottom face on the outer surface of container body 1. Mounting electrodes 4 are electrically connected to the crystal-connecting terminals through conductive paths or the like formed in the laminated ceramics.
Crystal blank 2 is, for example, an AT-cut quartz crystal blank. The AT-cut crystal blank has a resonant frequency determined by its thickness. As illustrated in FIG. 2, crystal blank 2, which is substantially rectangular in shape, is formed with excitation electrodes 5 on both main surfaces thereof such that they oppose each other. Extending electrodes 6 are extended from excitation electrodes 5 toward both ends of one side of crystal blank 2, respectively. Then, both ends of the one side of crystal blank 2, to which extending electrodes 2 are extended, are secured to crystal-connecting terminals on the inner surface of container body 1 by conducive adhesive 7. In this way, crystal blank 2 is held horizontally with respect to container body 1, and electrically and mechanically connected to container body 1.
Cover 3 is made, for example, of a flat metal plate, and bonded on the top face of frame wall 1b of container body 1, for example, by seam welding. In this event, frame wall 1b is previously provided with a metal ring or a metal film (not shown) for welding on the top face. Cover 3 is bonded on the top face of frame wall 1b to hermetically encapsulate crystal blank 2 in the recess.
However, in the conventional surface-mount crystal unit described above, as the outer planer size is increasingly reduced, container body 1 has a smaller effective area of the inner surface. This leads to a reduction in size of crystal blank 2 contained in container body 1. Even if crystal blank 2 used herein is an AT-cut crystal blank, the resonant frequency of which is determined by its thickness, crystal blank 2 exhibits higher oscillation characteristics and electric characteristics as it has a wider planar area, resulting in increased degrees of freedom in designing. Stated another way, the crystal blank is degraded more in the oscillation characteristics as its planar area is smaller, possibly causing an increase in crystal impedance (CI) and occurrence of spurious oscillations. Thus, the crystal blank having a smaller planar area exhibits lower electric characteristics and damages the degree of freedom in designing.
To overcome the foregoing problem, it is contemplated to reduce thickness d of frame wall 1b of concave container body 1 in order to increase the planar area of crystal blank 2 while maintaining the outer dimensions of the crystal unit. However, since this strategy involves a degraded strength of container body 1 and difficulties in manufacturing, thickness d of frame wall 1b cannot be reduced beyond a certain limit. Thus, concave container body 1 made of laminated ceramics has limitations in maintaining an effective internal area for containing the crystal blank therein while promoting a reduction in the outer dimensions thereof. For reference, assuming a crystal unit having the outer dimensions of 2.5 mmxc3x972.0 mm, frame wall 1b must have a thickness of approximately 0.35 mm, so that container body 1 has an effective internal area of 1.8 mmxc3x971.3 mm (2.34 mm2). Since crystal blank 2 must be smaller than at least this area, the resulting crystal unit will experience difficulties in meeting specifications associated with CI, spurious oscillation and the like.
It is therefore an object of the present invention to provide a surface-mount crystal unit which is capable of maintaining an effective internal area wide enough to contain a crystal blank to exhibit satisfactory electric characteristics even if its outer planar dimensions are reduced.
The object of the present invention is achieved by a surface-mount crystal unit which has a planar substrate having a pair of crystal-connecting terminals on a first surface and mounting terminals on a second surface, a crystal blank having a pair of extending electrodes which is formed on the outer peripheral portion thereof and is secured to the crystal-connecting terminals with a conductive adhesive to hold the crystal blank on the planar substrate, a metal film formed along the outer periphery of the first surface, and a concave metal cover having an open end face, wherein the open end face is bonded to the metal film to hermetically encapsulate the crystal blank between the metal cover and the planar substrate.
Specifically, focusing attention on the requirement of a thick frame wall resulting from a crystal blank contained in a concave container body made, for example, of laminated ceramics, the present invention solves the problem in the conventional surface-mount crystal unit described above by substituting a planar substrate for the concave container body, and bonding to the planar substrate a concave metal cover which maintains a sufficient strength even with a small thickness. According to the present invention, since the thickness of the metal cover can be made smaller than the thickness of the frame wall made of laminated ceramics, an effective internal area can be increased for containing a crystal blank. As a result, a crystal blank having a large planar area can be used so that a resulting surface-mount crystal unit, even if reduced in outer dimensions, can maintain satisfactory electric characteristics. It should be noted that although there are known surface-mount crystal units which employ a planar substrate made of ceramic and a concave cover made of ceramic, a surface-mount crystal unit having a planar substrate and a concave cover made of a metal is not at all found on the market.